Daern's Instant Fortress (DMG 160) is ordinarily a 1-inch metal cube, but rapidly grows into a 20x20x30 ft fortress when activated, dealing 10d10 bludgeoning damage to anything in its way.

What if the 10d10 damage is not enough to clear its space? For example, what would happen if a player tried to activate the Fortress deep inside a small cave? Would the item fail to activate, or would it take damage/be destroyed, or would it burst through the walls of the cave?

While this question is specifically about the Instant Fortress, I'm also interested about a general rule for this circumstance. Some items, like the Figurines of Wondrous Power, have an explicit restriction on changing size in enclosed spaces, whereas others, like the Staff of the Woodlands, do not. Is there a general way to reconcile these inconsistencies, or do I just have to make a ruling?

\$\begingroup\$@Rekesoft Please don't leave mini answers in comments. We do not support answers in comments because comments do not support features like proper voting and the wiki-style editing that allow us to vet, correct, and improve the content.\$\endgroup\$
– SevenSidedDieJun 20 '17 at 17:23

5 Answers
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Objects in the area that aren’t being worn or carried take this damage and are pushed automatically.

Your small cave's walls get pushed away to accommodate the fortress.

As for generalizing, you can't: at least not from an item that has a specific rule. Generally, generalizing (heh!) would need a general premise to start from and I don't believe that there is one - these types of items are either specific about what happens or leave it unsaid and therefore up to the DM.

\$\begingroup\$This seems to be a great tool for tunneling then!\$\endgroup\$
– lucidbrotJun 20 '17 at 6:24

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\$\begingroup\$I'm not sure terrain features are usually thought of as "objects" in this fashion. As lucid says, this then becomes the clever meta way to create tunnels, level terrain, wreck buildings, etc. Which means the item has now been granted an entirely new degree of functionality it was probably not meant to have.\$\endgroup\$
– zibadawa timmyJun 20 '17 at 13:40

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\$\begingroup\$@zibadawatimmy " an entirely new degree of functionality"? Say rather "a clever way to create tunnels that was always inherent in the item's functions"\$\endgroup\$
– Dale MJun 21 '17 at 7:22

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\$\begingroup\$The term "object" is defined on DMG p. 246 (also stated here): "For the purpose of these rules, an object is a discrete, inanimate item like a window, door, sword, book, table, chair, or stone, not a building or a vehicle that is composed of many other objects."\$\endgroup\$
– V2Blast♦Mar 27 '18 at 0:00

In one of the Drizzt Do'Urden novels we see one in use; in the Underdark which is all caves and underground tunnels when the mage who owned it decided to set it up somewhere new it punched through the roof and doesn't damage the tower.

Of course there are multiple layers of tunnels that make up the Underdark but it does give a sample that's directly related to the matter at hand.

As always your mileage may vary, but in my considered opinion from the evidence at hand that cave is screwed up when you activate the tower.

It breaks and collapses

According to the DMG (page 161):

The roof, the door, and the walls each have 100 hit points, immunity to damage from nonmagical weapons excluding siege weapons, and resistance to all other damage.

The fact that the tower has hit points stats and that it is not immune to, say, a trebuchet throwing boulders at it means that if it crashes against a rock much, much larger than anything the largest trebuchet in the world could ever manage to throw, such as a whole mountain, it's destroyed on impact.

As a houserule, I could support @DaleM's interpretation if the cave was a hole underground, not too deep, and the walls and ceiling were not pure rock, but more like earth or sands, but as a general rule, the tower can suffer damage and so it would be destroyed when hitting the walls.

\$\begingroup\$-1 For two reasons. 1.) A stationary cave ceiling or wall is not a siege weapon. 2.) That section is clearly indicating attacks after it has deployed, the question asks what happens during deployment.\$\endgroup\$
– T.J.L.Jun 22 '17 at 20:43

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\$\begingroup\$@T.J.L. Mixing magic and physics is always going to end badly, but still, you have to make some assumptions in cases like this. The cave walls are not moving, surely, but the siege is expanding from a 1 inch cube to a large building, shoving objects around. So, it's moving. If I punch you in the nose it's the same as if you headbutt my fist with your nose. As for the point 2, the problem is, the manual doesn't say anything about the case we're arguing about. Which is the reason of the question.\$\endgroup\$
– RekesoftJun 23 '17 at 7:19

It is your world, so you can always tweak as you will, but the fact it deals damage and is made out of adamantite, would make the easiest choice The cave warps to fit the dimensions. As for implying a damage to the tower, that too is up to you, but the text of the item grants it immunity to non magical damage, bludgeoning in this case, except in the case of siege weapons, which are a class of objects, and a trait on certain huge and gargantuan creatures.

This really falls entirely on the DM, the item description does not support any particular interpretation.

As a DM I had players try something similar, in their case a cave wide enough but not tall enough for the fortress, and I ruled the roof of the fortress took 10d10 damage as it slammed into a solid stone ceiling,(it was not destroyed but was close). It did expand to it full size and carve out the cave but got heavily damaged in the process.

My justification was slamming in to an essentially immovable mass of solid stone is similar in effect to a siege engine ram, so it should damage it while at the same time I used the fortess's own damage since it is supplying the force. If you feel this is a bit harsh you always could add the fortress's resistance to damage, having sections take only half of 10d10. This also means you are not penalizing thinking outside the box but encourages actually thinking.